What this assumes
Calculates volume from length × width × depth and converts to cubic yards (27 cubic feet per yard). A waste allowance (default 5%) is added. Costs use representative US/Canada figures from an editable data file — ready-mix per cubic yard, bag yield and bag cost, or gravel per cubic yard — and vary a lot by region, supplier and date.
This is an estimate to help you plan, not financial, tax or legal advice.
Getting the volume right
The single most useful thing this calculator does is stop you running short. Concrete is unforgiving — once a pour starts, you can’t pause it and pop to the merchant for another bag. So it works out your volume, adds a sensible waste allowance, and tells you what to order.
The maths itself is simple: length × width × depth, all in feet, gives cubic feet; divide by 27 to get cubic yards, which is how concrete and gravel are usually sold. A 20 ft × 10 ft slab at 4 inches deep works out at about 2.6 cubic yards once you add waste.
Bags or ready-mix?
For small jobs — setting fence posts, a little pad — bags you mix yourself are fine. For anything bigger, ready-mix delivered by truck is normally cheaper per yard and an enormous amount less effort, though suppliers often have a minimum order. The calculator estimates either, so you can compare.
A note on cost
The cost figures are representative and vary a lot by region, supplier and time of year. Use them to get in the right ballpark, then confirm with a local quote — especially for ready-mix, where delivery and minimum-order charges matter.
Planning the wider project
If the concrete is for fence posts, the fence cost calculator works out the posts, panels and overall cost so the two estimates line up. Measure twice, order once, and give yourself that little bit of spare.
Planning estimate only. Material costs and yields differ between products and regions — always check the bag or supplier specification and a local quote.